Telecommunications system and receiver

ABSTRACT

The invention relates to a method for estimating power in a CDMA telecommunications system, in which method a sample set is formed ( 300 ) from the received signals. The proposed method reiterates ( 302  to  312 ) the following steps, until a predetermined ending condition is fulfilled: forming ( 302 ) an absolute value set from the absolute values of sample set elements, setting ( 304 ) a threshold value; calculating ( 306 ) a reference value by multiplying the mean of the absolute value set by the threshold value; comparing ( 308 ) the element of the absolute value set with the reference value and forming ( 312 ) a second sample set by deleting elements exceeding the reference value from the sample set; the ending condition being fulfilled, estimating ( 314 ) the power on the basis of the remaining sample set.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to CDMA telecommunications systems and methods andreceivers employed therein for power estimation.

2. Description of the Related Art

In CDMA (Code Division Multiple Access) telecommunications systemsestimation of received power is important, for instance, in powercontrol and handover decisions. Estimation of an average signal power isimportant in the optimal adjustment of system parameters. The signalpower estimation is also important in view of the handover algorithm.

Particularly in environments, where rapid, impulsive interferences,resulting from multipath propagation, for instance, affect the qualityof a received signal, it is important to estimate the power accurately.Impulses disturb the methods used for power estimation, andconsequently, the power estimation may fail. The power estimationalgorithms used in the CDMA telecommunications systems try to filter offfast impulses from the received signal, and ideally they should besimple.

It has become more and more common to use so-called robust, i.e.control-weighted methods for eliminating impulsive interference in thetelecommunications systems. The robust methods are not sensitive to bigchanges in individual observation values, such as impulse-likeinterference in a received signal. The prior art robust methods utilizeso-called order statistics, the basic idea of which is to detect andeliminate observed interference by properties associated with anobservation set arranged on the basis of variable values. One prior artrobust method is so-called median-type filtering. This kind of prior artmedian-type filtering, used for power estimation, is described ingreater detail, for instance, in the publication by C.Tepedelenlio{hacek over (g)}lu, N. Sidiropoulos, G. B. Giannakis,“Median Filtering For Power Estimation In Mobile Communication Systems”,Third IEEE Signal Processing Workshop on Signal Processing Advances inWireless Communications, Taoyuan, Taiwan, Mar. 20–23, 2001, pp 229–231.

The prior art solutions have a drawback that advance information on aninterfering signal is required in order that correct interferenceimpulses could be detected. The prior art solutions do not operate wellat high impulse interference frequencies, so they are not very efficientcomputationally. In addition, the prior art solutions are difficult toimplement in practice.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and a receiverimplementing the method such that drawbacks associated with the priorart can be reduced. This is achieved with a method for estimating powerin a CDMA telecommunications system, in which method a sample set isformed of the received signals. In the method of the invention, thefollowing steps are reiterated until a predetermined ending condition isfulfilled: forming a set of absolute values from the absolute values ofsample set elements; setting a threshold value; calculating a referencevalue by multiplying the mean of the absolute value set by the thresholdvalue; comparing the element of the absolute value set with thereference value and forming a second sample set by deleting elementsexceeding the reference value from the sample set; the ending conditionbeing fulfilled, estimating the power on the basis of the remainingsample set.

The invention also relates to a receiver comprising means for forming asample set from signals received. With the means, the receiver of theinvention is arranged to reiterate the following until a predeterminedending condition is fulfilled: to form a set of absolute values from theabsolute values of sample set elements, to set a threshold value; tocalculate a reference value by multiplying the mean of the absolutevalue set by the threshold value; to compare the element of the absolutevalue set with the reference value; and to form a second sample set bydeleting elements exceeding the reference value from the sample set, thereceiver is further arranged to estimate the power on the basis of theremaining sample set, when the ending condition is fulfilled.

The preferred embodiments of the invention are disclosed in thedependent claims.

Several advantages are achieved with the method and system of theinvention. A solution that is computationally efficient and simple toimplement will be achieved. The proposed solution does not require anyadvance information on interference. The proposed solution also has anadvantage that the method works even at very high frequencies of impulseinterference.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail inconnection with preferred embodiments, with reference to the attacheddrawings, in which

FIG. 1 shows an example of one telecommunications system according tothe proposed solution;

FIG. 2 shows an example of a receiver according to the proposedsolution;

FIG. 3 shows a block diagram of a power estimation method according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the invention can be applied to CDMAtelecommunications systems, which comprise one or more base stations anda plurality of terminal equipment, which communicate with one or morebase stations. One of the most important functions of the equipment in aCDMA-based system is transmission power control. As distance grows, thesignal strength attenuates, and a mobile station in the vicinity of abase station overpowers signals of more remote base stations. Theoptimal operation of the system requires that the mobile stations adjusttheir transmission power such that the signals arriving at the basestation are as equal in power as possible. The base stations must alsotransmit their data in correct sequences without that they deviate intime.

The structure of a telecommunications system can be in essence as shownin FIG. 1. The telecommunications system comprises a base station 100and a plurality of generally mobile subscriber stations 102 to 106,which have bi-directional connections 108 to 112 to the base station100. The base station 100 switches the connections of the terminalequipment 102 to 106 to a base station controller 114, which forwardsthem to other parts of the system and to a fixed network. The basestation controller 114 controls the operation of one or more basestations. The base station controller 114 monitors the quality of theradio signal and the transmission power, and takes care of the handoverof the mobile station. In addition to the electronic components requiredfor transmitting and receiving radio communication, the base station 100also comprises signal processors, ASIC circuits and general-purposeprocessors, which take care of data transmission to the base stationcontroller 114 and control the operation of the base station 100. Thebase station 100 may comprise one or more transmitter/receiver units.The receiver of the proposed solution can be placed in connection witheither the base station 100 or the mobile subscriber stations 102 to106.

FIG. 2 shows an example of a receiver 200 according to the proposedsolution. The receiver comprises an A/D converter 202, a powerestimation block 204, an adapted filter 206 and a control unit 208. Thereceiver may also comprise other means implementing receiver operations,such as speech and channel encoders, modulators and RF parts. Inaddition, the device comprises an antenna 201, by means of which signalsare transmitted and received.

All operations of the receiver 200 are controlled by the control unit208, which is typically implemented by means of a microprocessor andsoftware or separate components. The A/D converter 202 converts thecontinuous signal received by the receiver 200 into a digital form. Theadapted filter 206 is a specific filter adapted to let only the desiredsignal pass through with slight signal attenuation and to block allother waveforms (including noise). Prior to the adapted filter 206 thesignals are at chip level and after the adapted filter at symbol level.The power estimation block 204 is implemented by means of ASIC circuitsor a microprocessor and software. In the proposed solution it ispossible to place also other components between the power estimationblock 204 and the adapted filter 206.

In FIG. 2, sampling, controlled by the control unit 208, in the A/Dconverter 202 takes place by reading the value of a continuous signal atgiven intervals. This sampling interval is generally constant indigitizing each signal. After digitizing the signal, the estimationblock 204 performs operations controlled by the control unit 208. In theexample presented, first, a sample set is formed from the signalsreceived from the A/D converter 202, and from the absolute values of thesample set elements is formed a set of absolute values. In theestimation block 204 there is also determined a reference value, whichis the product of the mean of the absolute value set and a predeterminedthreshold value. Next, in the estimation block 203 there is performed acomparison between the values of elements of the absolute value set andthe reference value, whereafter the estimation block 204 deletes suchelements from the sample set that exceed said reference value. Theestimation block 204 repeats the above-mentioned steps until no elementsexceeding the reference value are found in the absolute value set, oruntil a predetermined number of reiteration rounds have been performed.Thereafter, power estimation is carried out in the estimation block 204on the basis of the remaining sample set.

FIG. 3 shows a block diagram of a power estimation method according tothe proposed solution. In step 300, a sample set is formed from thereceived signals. The received signal comprises thermal noise andinterference impulses. The object of the method is to cancel theimpulsive interference from the received signal prior to determining thepower of the received signal. This is implemented by dividing thereceived chip-level samples into a desired set and an interfering set,which is done by means of a threshold value. The desired set will thusbe the remaining sample set, from which the interferences have beencancelled. The sample set formed in step 300 is assumed to be Gaussianwith zero mean, whereby the amplitude of the sample sequence isRayleigh-distributed. This situation is achieved in multiple userDS-CDMA systems, in which the power control works. Whereas, if there areonly few users, or the power control does not work, the mean of thevariables deviates from zero, whereby the amplitude is Rice-distributed.

After forming the sample set in step 300, the process proceeds to step302, in which an absolute value set is formed from the absolute valuesof the sample set elements. As the desired set of chip-level samples isassumed to be Gaussian with zero mean, the absolute value set consistingof the absolute values of the desired set is Rayleigh-distributed. Inpractice, the desired set is not precisely Gaussian, but the method issimplified by this assumption. The aim of the so-called robust methodsis not to find a real model of an adjustable system per se, but the aimis to achieve sufficiently good functioning of the system as a whole.For instance, if the sample set comprises a DS signal and thermal noise,the absolute values of these samples are Rice-distributed. The proposedmethod can still be used in that case, too.

In step 304, a threshold value is set that is needed in the subsequentstep 306 for computing a reference value. The threshold value requiredfor the reference value is obtained from Rayleigh distribution. Forinstance, if it is desired that 0.1 percent of the sample set elementsbe deleted, the threshold value is advantageously 2.97. Whereas, if itis desired that 1 percent of the sample set elements be deleted, thethreshold value is 2.42. The assumption that the desired set is Gaussianwith zero mean is sufficient for determining the threshold value. Thethreshold value 2.97 is an acceptable value in all desired casesindependent of Gaussian set variance. In a case of non-interference thisparticular threshold value 2.97 causes a situation, in which only 0.1percent of the samples of the desired set is erroneously selected to beimpulses.

After setting the threshold value in step 304, the process proceeds tostep 306, in which a reference value is computed by multiplying the meanof the absolute value set by said predetermined threshold value. Next,in step 308 the element of the absolute value set is compared with thereference value. In step 310 it is checked whether a given ending valueis fulfilled. This is carried out, for instance, by searching if thereare values among the element values of the absolute value set thatexceed the reference value. If values exceeding the reference value arenot found, the ending condition is fulfilled and the process proceeds tostep 314 to estimate power. Whereas, if it is detected in step 310 thatthere are values among the absolute value set that exceed the referencevalue, the ending condition is not fulfilled and step 312 is proceededto, in which the elements exceeding said reference value are deletedfrom the sample set. From step 312 the process proceeds back to step302, in which the absolute value set of the remaining sample set iscomputed. Thereafter, the process proceeds again stepwise, until in step310 no element values exceeding the reference value are found, or untilsteps 302, 304, 306, 308, 310 and 312 have been reiterated apredetermined number of times. Thus, the ending condition can befulfilled, for instance, when element values exceeding the referencevalue are no longer found, or, when steps 302, 304, 306, 308, 310 and312 have been reiterated a predetermined number of times.

Power estimation in step 314 is performed, for instance, by using aso-called classical power estimator, whereby the power estimation isperformed on the basis of the mean of squares of the absolute values ofthe elements of the desired remaining sample set by using formula (1),or on the basis of the mean of squares of the elements of the absolutevalue set of the desired remaining sample set by using formula (2):

$\begin{matrix}{P_{clas} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}\;{y_{i}}^{2}}}} & (1) \\{{P_{clas} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}x_{i}^{2}}}}\mspace{31mu}} & (2)\end{matrix}$

-   -   wherein:    -   P_(clas) is power,    -   N is the number of elements,    -   |y_(i)| is the absolute value of the element of the desired        remaining sample set,    -   x_(i) is an element of the absolute value set of the desired        remaining sample set.

Power estimation in step 314 can also be carried out by a method that isbased on Rayleigh distribution, whereby the power estimation isperformed by means of formula (3):

$\begin{matrix}{P_{ray} = {\frac{4}{\pi}( {\frac{1}{N}{\sum\limits_{i = 1}^{N}x_{i}}} )^{2}}} & (3)\end{matrix}$

wherein:

P_(ray) is power and the mean of the elements of the absolute value setshown in brackets has already been computed in step 306.

The built-in property of the proposed method, the ready-computed mean,enables more simplified power determination as compared withconventional methods. The method assumes that the desired setconstitutes Gaussian distribution with zero mean. In addition, varianceof the desired set need not be known on determining a threshold value.Thus, the desired set can be e.g. a direct sequence signal, which isbelow zero mean Gaussian noise (thermal noise). This situation arises,for instance, in spread spectrum systems, in which the signal-to-noiseratio at chip level is less than zero desibel. After the adapted filter206, the signal-to-noise ratio is higher, depending on the length of thespreading code, i.e. detection amplification. The method does notrequire any advance information on the interference, however.

The proposed solution works well at a very high impulse interferencefrequency, for instance with a frequency value of 0.5. The proposedsolution works the better the lower the impulse interference frequency.An error in power estimation is less than 30 percent with all impulsepowers and an error in all impulse frequencies is less than 0.2. Themethod works well also in interference-free situations. The proposedmethod works the better the lower the frequency of the impulse noise,irrespective of the power of the impulse noise. Thus, the method canalso cancel low-frequency interference. The method works also even if upto 50 percent of the samples were interference impulses.

Even though the invention is described above with reference to theexample of the attached drawings, it is apparent that the invention isnot restricted thereto, but it can be modified in a variety of wayswithin the inventive idea of the accompanying claims.

1. A method for estimating power in a CDMA telecommunications system,the method comprising: forming a sample set of the received signals;reiterating the following steps until a predetermined ending conditionis fulfilled: forming an absolute value set from the absolute values ofsample set elements, setting a threshold value, calculating a referencevalue by multiplying the mean of the absolute value set by the thresholdvalue, comparing the element of the absolute value set with thereference value, and forming a second sample set by deleting elementsexceeding the reference value from the sample set; the ending conditionbeing fulfilled, estimating the power on the basis of the remainingsample set.
 2. The method of claim 1, wherein the threshold value is seton the basis of Rayleigh distribution.
 3. The method of claim 1, whereinthe ending condition is fulfilled, when the steps have been reiterated apredetermined number of times.
 4. The method of claim 1, wherein theending condition is fulfilled, when the absolute value set does notinclude element values that exceed the reference value.
 5. The method ofclaim 1, wherein the power is estimated on the basis of the mean ofsquares of the absolute values of the remaining sample set elements. 6.The method of claim 1, wherein the power is estimated by the Rayleighdistribution method.
 7. A receiver in a CDMA telecommunications system,the receiver comprising means for: forming a sample set from signalsreceived; reiterating the following steps until a predetermined endingcondition is fulfilled: forming an absolute value set from the absolutevalues of the sample set elements, setting a threshold value,calculating a reference value by multiplying the mean of the absolutevalue set by the threshold value, comparing the element of the absolutevalue set with the reference value, and forming a second sample set bydeleting elements exceeding the reference value from the sample set; thereceiver further comprises means for estimating the power on the basisof the remaining sample set, when the predetermined ending condition isfulfilled.
 8. The receiver of claim 7, wherein the receiver comprisesmeans for setting a threshold value from Rayleigh distribution.
 9. Thereceiver of claim 7, wherein the predetermined ending condition includesfulfilment of a predetermined number of reiterations.
 10. The receiverof claim 7, wherein the ending condition includes a comparison arrangedfor the receiver, in which comparison no element values exceeding thereference value are found.
 11. The receiver of claim 7, wherein thereceiver comprises means for estimating the power on the basis of themean of the squares of the absolute values of the sample set elements.12. The receiver of claim 7, wherein the receiver comprises means forestimating the power by the Rayleigh distribution method.